The use of heavy metal collimators of circular shape is now well known for stereotactic radiosurgery using treatment planning machines such as linear accelerators (LINACs) as an X-ray source (see the XKnife information from Radionics, Inc., Burlington, Mass.). Circular collimators are used made of lead or Cerrobend heavy metal with circular apertures of different sizes to collimate the X-ray beams from a LINAC. A collimator is rotated in a so-called gantry angle and couch angle around an isocenter at which position is located a target volume within the body of a patient. Conformal stereotactic radiosurgery involves use of irregularly shaped collimators that are typically non-circular. These may be so-called cut-block collimators, multi-leaf collimators, or miniature multi-leaf collimators (see the information from Radionics, Inc., Burlington, Massachusetts or Fischer GmbMH, Frieburg, Germany). Conformal collimators are usually used in a static mode, meaning static discrete beam directions are determined and different collimators shapes are used depending on the shape of the target volume such as a tumor in the patient's head. Circular collimators are usually used in an arc mode, which means that the circular collimator is swept over the patient's head through the couch and gantry angles. A certain degree of target volume dose shaping is achieved by circular collimator arc therapy, but this is limited because of the limitation in shapes of the circular collimators. More conformal collimation is achieved by the cut-block or multi-leaf changeable shape collimators, but these are complicated devices and are labor intensive to make for a specific patient. In general, the system of the present invention is directed at an improved system for accomplishing conformal arc therapy for LINAC radiosurgery in the body. The system offers a simple and practical way of improving the dose distribution of X-rays for an irregularly shaped target volume by a combination of circular collimators and collimator blocking jaws which can be used to eclipse a portion of the circular beam aperture of the circular collimator.
Heavy metal blocking jaws are typically used in the heads of the linear accelerator to provide large field blocking for standard radiotherapy irradiation of X-rays. Typically, a set of two pairs of opposing jaws orthogonally oriented to each other and moveable in an orthogonal direction to the beam direction are present in the gantry head of a typical X-ray LINAC. These jaws alone are normally not adequate to perform stereotactic radiosurgery. The penumbra effects of use of the four jaws in a LINAC combined with arc therapy would not provide sufficient tightness of radiation for small to medium size brain tumors for instance to be effective for radiosurgery and are typically not employed for such application in radiosurgery. Use of the straight jaw and circular collimator configuration are disclosed herein together with treatment planning software to accommodate its use for conformal arc radiosurgery.